The production of cycloalkylaromatic compounds, such as cyclohexylbenzene, is a commercially important reaction since the latter has potential as a source of phenol and cyclohexanone, which are important intermediates in the chemical industry with utility in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process via cumene. This is a three-step process involving alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone. However, the world demand for phenol is growing more rapidly than that for acetone. In addition, the demand for propylene is likely to increase. Thus, a process that does not require propylene as a feed and coproduces higher ketones, rather than acetone, may be an attractive alternative route to the production of phenol.
One such process involves the catalytic hydroalkylation of benzene to produce cyclohexylbenzene, followed by the oxidation of the cyclohexylbenzene (analogous to cumene oxidation) to cyclohexylbenzene hydroperoxide, which is then cleaved to produce phenol and cyclohexanone in substantially equimolar amounts.
An example of such a process is described in, for example, U.S. Pat. No. 6,037,513, in which an aromatic hydrocarbon, such as benzene, is contacted with hydrogen in the presence of a bifunctional catalyst, which has both hydrogenation activity and alkylation activity. In particular, the catalyst comprises a metal having hydrogenation activity, such as palladium, and a crystalline inorganic oxide material having alkylation activity and an X-ray diffraction pattern including d-spacing maxima at 12.4±0.25, 6.9±0.15, 3.57±0.07 and 3.42±0.07 Angstrom. The catalyst is produced by impregnating the crystalline inorganic oxide material with an aqueous solution of a palladium salt. The impregnated sample is then dried and calcined in air. Then prior to employing the catalyst in a hydroalkylation reaction, the catalyst is treated with 50 cc/min of flowing hydrogen for 2 hours at 300° C. and 1 atmosphere pressure. Although not stated in the '513 patent, this hydrogen treatment is employed to activate the catalyst by converting the palladium oxide produced in the calcination step to a more active form of palladium.
One problem of producing phenol via benzene hydroalkylation is the selectivity of the catalyst to cyclohexylbenzene since the process can produce significant quantities of unwanted by-products, particularly cyclohexane and heavies, such as dicyclohexylbenzene. Although many of these impurities can be removed by down-stream processing steps or further conversion processes such as transalkylation of dicyclohexylbenzene with benzene, such steps necessarily add cost to the overall process and hence there is significant interest in improving the cyclohexylbenzene selectivity of the hydroalkylation process.
According to the present invention, it has now been found that the cyclohexylbenzene selectivity of hydroalkylation catalysts, such as those disclosed in U.S. Pat. No. 6,037,513, can be improved by lowering the temperature of the hydrogen pre-treatment used to activate the catalyst. Lowering the pre-treatment temperature not only improves the catalyst selectivity but reduces the cost and time required to affect the pre-treatment process.